Silicon (Si) is the second most abundant element in the earth's crust and exists in rocks mostly in the form of silica. Silica has various uses. For example, it can be used to produce utensils, optical lenses, optical components, and decorative artificial jewels. It can also be added to a coating material to provide the coating material with a hydrophobic property so as to permit the coating material to possess water-proof and/or seepage-proof effects.
The hydrophobic property of the coating material may be further enhanced by adding nano-sized silica particles to the coating material. Uniformity of the hydrophobic property of the coating material added with the nano-sized silica particles depends on the particle size distribution of the nano-sized silica particles. When the particle size distribution of the nano-sized silica particles is narrow, the hydrophobic property of the coating material added with the nano-sized silica particles is relatively uniform. It has been found in the art that the hydrophobic property of the coating material may be further enhanced when the silica particles have a particle size distribution ranging from 20 nm to 40 nm.
Silica nanoparticles may be prepared by various methods. For example, U.S. Pat. No. 7,799,303 B2 discloses a method for preparing silica nanoparticles from siliceous mudstone, which includes: heat-treating the siliceous mudstone to remove water; leaching the siliceous mudstone with hydrochloric acid to remove impurities, thereby forming a silica constituent; leaching the silica constituent with sodium hydroxide, thereby forming a sodium silicate aqueous solution; passing the sodium silicate aqueous solution over an ion-exchange resin, thereby producing a silicate aqueous solution having a sodium concentration of 50 ppm or less; and flame spray pyrolyzing the silicate aqueous solution with a flame reactor to prepare nanoparticles from the silicate aqueous solution.
Although the silica nanoparticles may be prepared by the method disclosed in U.S. Pat. No. 7,799,303 B2, the particle size distribution of the silica nanoparticles thus prepared is relatively wide. Therefore, the hydrophobic property uniformity of the coating material added with the silica nanoparticles thus prepared may be unsatisfactory.
It is desirable in the art to provide a method for preparing silica nanoparticles having a relatively narrow particle size distribution.